There has been a continual trend in microelectronics manufacturing towards single-wafer processing for a variety of semiconductor fabrication steps. One requirement inherent in the design of single-wafer reactors is the need to unobtrusively secure the wafer during processing, while at the same time controlling the temperature of the wafer and temperature uniformity across the surface of the wafer. The ability to control the temperature, and temperature uniformity, of a silicon wafer is of great importance in a wide variety of semiconductor processing techniques since the processing generally involves metallurgical or chemical reactions which may be significantly affected by the temperature of the surface on which the processing is carried out.
Mechanical wafer clamps which engage a portion of the wafer front surfaces where processing is to be performed can create process uniformity problems by interfering with flow of gas, altering the plasma distribution, and acting as a heat sink. If improperly designed, mechanical wafer clamps can also cause the formation of particulates with consequent contamination of the wafer and contribute to bowing of the wafer with consequent complications of focus and registration for lithographic processes as well as planarization processes.
An electrostatic chuck (ESC), which uses an electrostatic potential to hold a wafer in place during processing, can avoid the problems of heat sinking and interference with gas flow at the front surface of the wafer by providing contact with only the back side of the wafer. Therefore, use of an electrostatic clamp is an attractive alternative to front side mechanical clamps. Consequently, in recent years, there has been a considerable interest in use of an electrostatic chuck which, in addition to the above advantages, can also reduce the tendency of bowing and contamination of the front side of the wafer over mechanical clamps or gravitational wafer placement (see, for example, G. A. Wardly, Rev. Sci. Instrum., 44, 1506 (1973)).
However, electrostatic chucks also constitute heat sinks at the back side of the wafer and, although to a lesser degree than mechanical chucks, have contributed to a lack of temperature uniformity of the wafer. To ensure homogeneous film processing over the entire area of the wafer, a substantially uniform wafer temperature must be produced and maintained at the wafer surface. The rate of film deposition, the physical, electrical, and optical properties, and the composition of the deposited material can all be affected by the temperature of the wafer during the deposition process. Likewise, the rate of etch, the selectivity of etch and anisotropy of etch can be affected by the temperature of the wafer during plasma etching.
The control of heat transfer between the wafer and wafer holder or chuck, regardless of type, is particularly complex in plasma systems which operate at low pressure and have an RF bias applied to the wafer. Thermal energy is transferred to the wafer surface through ion bombardment, and the chuck is ideally required to remove large amounts of heat from the wafer while maintaining a stable and uniform temperature at the wafer surface. In these systems, as exemplified by U.S. Pat. Nos. 4,261,762 and 4,680,061, a gas (usually He) is employed between the wafer and the chuck to control the removal of heat from the wafer. Since many semiconductor processing operations are carried out at extremely low pressures, the pressure of gas between the wafer and the chuck must often be at a greater pressure in order to provide an adequate degree of heat transfer. This greater pressure, of course, tends to separate the wafer from the chuck. Consequently, some type of wafer clamping (e.g., mechanical or electrostatic) is required.
During many types of semiconductor processing and plasma processes, in particular, it is found that the wafer temperature is significantly higher than the chuck temperature and that control of thermal resistances across the wafer/ESC interface is critical in controlling wafer temperature uniformity. More specifically, the thermal conduction within a semiconductor wafer (e.g., across its thickness) and within the body of a wafer chuck are generally well behaved and predictable. However, the interface between the semiconductor wafer and chucks of both the mechanical clamping type and the electrostatic type has appeared to be highly unpredictable as well as imposing a substantial degree of complexity on the numerous heat transfer mechanisms involved. For example, since it is common practice to circulate an inert gas, such as helium, as referred to above, through circumferential grooves in the face of the chuck, heat transfer coefficients for both the gas and the chuck surface must be considered. These heat transfer coefficients are markedly different and each can also vary widely. Additionally, the relative contributions of heat transfer across the interface also vary with local and overall contact fractions between the wafer and chuck surface.
The design of the pattern of grooves formed in the face of a chuck has heretofore been based principally on the achievement of a particular contact ratio with a radially symmetrical pattern. Difficulty of measurement of temperature at small areas of the surface of a wafer has largely prevented refinement of designs. However, significant variations of processing have been detected between different locations on a processed wafer, inferring that temperature uniformity across the wafer face is not adequately maintained during semiconductor processing when held by any currently known type of chuck. Further, no convenient mechanism known for exercising temperature control or improving temperature uniformity has been available.